1. Field of the Invention
This invention relates to plastic closures with improved sealing means to seal containers that may have contents capable of generating pressure much higher than atmospheric pressure. In particular, it relates to plastic closures with pilfer-proof structures and to the manufacture of such closures.
2. The Prior Art
At the present time one of the most common types of closures for large beverage containers (about two liters) is made of sheet aluminum and is called a roll-on closure because of the way it is attached to a container. The neck of the container has a bead that extends outwardly at a region spaced about two cm. from the outermost end of the neck and forms a shoulder facing the body of the container. Between the bead and the outermost end of the neck is a thread that stops a short distance from the outermost end.
A roll-on closure is formed by punching out a disk from a sheet of aluminum and forming the disk so that its central part remains flat and is of approximately the same size as the end of the neck of a container of the type just described while its peripheral part is deformed into a generally cylindrical configuration that remains integrally attached to the perimeter of the central part and has an inner diameter approximately equal to the outermost part of the thread and bead on the neck of the container. The forming process also creates two knurled bands that provide gripping surfaces to facilitate removal of the closure after it has been attached to a container in the bottling process.
The designation "roll-on" is derived from the process of attachment in which the closure in the form just described is attached to the end of the neck of a filled container with the cylindrical part of the closure extending down past the threaded part of the neck. The central part is pressed firmly against the end of the neck, and the closure and container are rotated as a unit about their common axis while rollers that rotate about axes approximately parallel to the container axis are pressed against opposite sides of the cylindrical part at the region near the intersection between that part and the flat central part. The cross-sectional shape of the rollers is about the same as the valley in the thread on the neck of the container, and the radially inward pressure by the rollers as they roll along the valley of the rotating container deforms the cylindrical part of the closure to conform to the thread on the container.
The lowermost end of the cylindrical part extends below the bead on the bottle and is also subjected to inwardly directed deforming pressure by rollers that cause the edge of the lowermost end to fit tightly over the shoulder at the axial end of the bead facing the body of the container. This inwardly pressed end of the closure would prevent the closure from being unscrewed from the container except for the fact that, at some time in the formation of the closure, the cylindrical part is lanced by blades to form circumferential slits that almost sever the lowermost end from that portion of the cylindrical part that is deformed into a helical thread. The resulting inwardly pressed end, partially severed from the threaded part, is called the "pilfer-proof" because it prevents the contents of the container from being pilfered or tampered with without leaving a clear indication that the container has been opened. The threaded part of the closure, to which the pilfer-proof is attached, is called the skirt of the closure. The narrow parts that join the pilfer-proof to the skirt are called bridges, and they are defined by the ends of the slits formed by the lancing blades.
One way that the pilfer-proof can indicate tampering is by making the bridges so narrow or weak that they can be easily broken by the stretching force produced when the skirt moves upwardly as the closure in unscrewed. During this removal of the main part of the closure, the inwardly deformed edge of this type of pilfer-proof clings firmly to the downwardly facing shoulder of the bead on the neck of the container. The severed pilfer-proof then falls farther down on the neck and gives clear warning that the closure has been removed, or at least tampered with.
Another type of pilfer-proof, such as that illustrated in U.S. Pat. No. 3,824,921 to Hannon, has longitudinal slits that weaken its inwardly turned edge. When a closure having such a pilfer-proof is unscrewed, the resulting upward pressure causes the scores to rupture so that the sections of the inwardly turned edges can pivot outwardly as the result of camming action by the shoulder on the bead, thus indicating clearly that the closure has been tampered with. The bridges in such a structure are strong enough not to break before the scores rupture.
One of the supposed advantages of a roll-on closure is that it can be removed without the assistance of a mechanical opener, such as is required to remove the old-style crowns (which are still used on some containers). However, that advantage can only be obtained if the thread formed in the skirt is pressed in deeply enough to permit a strong enough longitudinal force to be generated by interaction between that thread and the container thread to sever or tear the pilfer-proof, according to the type of pilfer-proof. Unfortunately, the threads on closures are not always formed deep enough, and on occasion a closure with too shallow a thread will virtually explode off of the container and seriously injure the person trying to unscrew the closure. Such occasions are rare but not rare enough, and for safety, a measuring device of the type described in U.S. Pat. No. 4,135,306 by Charles N. Hannon should be used to measure the thread depth of roll-on closures.
Another supposed advantage of a roll-on closure is its ability to be screwed back onto the container to retain carbonation in the liquid in the container. Unfortunately, and in spite of the fact that a ring or disk of plastic is commonly used as a gasket or liner on the underside of the flat central part of the closure, it is difficult to twist the closure back on firmly enough to re-establish a gas-tight seal between the closure and the container.
For several years attempts have been made to form satisfactory closures by molding thermoplastic material, such as polyethylene or polypropylene or a co-polymer or other thermoplastic material, into a configuration that will seal and reseal a container. A number of types of such plastic closures have been proposed, and some of them have included pilfer-proofs. As in roll-on pilfer-proofs, plastic pilfer-proofs include a ring, gripping means to grip the container securely, and some weakened region that tears the first time the closure is removed from the container. If the closure has been tampered with to an extent that might adversely affect the contents of the container, the weakened region will be torn and thus indicate in two ways that such tampering has occurred: the tear in one or more places gives visible evidence of tampering, and the fact that, after the weakened region has been torn, the closure can be noticeably more easily removed gives tactile evidence.
In general plastic pilfer-proofs have fallen into one or the other of two categories. In one category, the pilfer-proof, or at least the ring, consists of material that shrinks as a result of being heated, which requires filled containers capped by such closures to pass through some sort of heating means, such as a heat tunnel. When everything works properly, the pilfer-proof shrinks around the neck of the container below the bead. Some of the pilfer-proofs of that category are arranged so that all of the bridges will be broken when the main part of the closure is unscrewed, leaving the pilfer-proof as a tell-tale ring on the neck of the container. Others of the heat-shrunk category are formed with a longitudinal groove that weakens the ring so that it will break as the closure is unscrewed and as most of the bridges are also breaking. However, one of the bridges 180.degree. away from the longitudinal groove is made stronger that the other bridges to hold the partially severed pilfer-proof attached to the skirt.
Pilfer-proofs that depend on heat shrinkage have a great disadvantage because heat tunnels are expensive and take up space in a bottling plant. In addition, the amount of heat must be carefully controlled so as to shrink the ring sufficiently without damaging the main part of the closure or the container or the contents of the container.
The other general category of pilfer-proofs, and the type with which this invention deals, is referred to as a mechanical type of pilfer-proof because its shape and resilience cause it to interlock with the container. Like heat-shrunk pilfer-proofs, mechanical pilfer-proofs have been proposed in which the weakened region is a plurality of slender bridges that attach the ring to the lower edge of the skirt. The gripping means consists of a number of projections integrally molded with the ring and arranged to project inwardly and upwardly toward the top of the closure.
In some mechanical pilfer-proofs, the projections are not formed so that they can be moved radially outwardly by the bead when the closure is being put on a container. As a result, either the projections must be compressible enough to be flattened by pressure between the ring and the bead on the neck of the container while the closure is being put on, or else the ring must stretch. Either of those requirements makes it difficult to put the closure on a container and makes it possible for the same resilience to allow the pilfer-proof to come off of the container without the necessary visible damage to the weakened region during removal.
In other types of mechanical pilfer-proofs, the plastic ring has been molded as a separate item and then welded to, or mechanically interlocked with, a separately molded closure, which can then be put on a container. Such a two-part structure requires that the closure be molded in one operation and then separately assembled with the pilfer-proof in a secondary operation that greatly increases the cost of the complete product.
In still other types of pilfer-proofs, the projections are molded as part of the ring, but they are not molded pointing upwardly as well as inwardly and must be folded into that position after the closures with such pilfer-proofs have been molded. The folding step is a secondary operation that adds to the production cost, and at least in some instances, projections thin enough and flat enough to be folded are not rigid enough to be sure that they will not unfold instead of tearing the bridges, as they should do.
One of the most difficult problems in molding gripping means that extend inwardly and at an angle toward the inner surface of the top of the closure is extracting the closures from a mold in which the gripping means are in the form of hooks, each of which has an inaccessible surface that faces generally upwardly and outwardly. One proposal that has been made in the past is to provide, for each such hook, a separate outer mold member that projects inwardly through the ring, itself, to form the inaccessible surface. Each of these mold members in such a complex arrangement must be separately withdrawn in its radial direction, for example, by cam action to allow removal of the molded closure after the elastomeric closure material has solidified.
Plastic closures can be made sufficiently deformable to preclude the necessity of a gasket, but some bottlers still want the liner so that plastic closures can be used in games of chance just like the roll-on closures. In addition, if the entire closure and its mechanical pilfer-proof are to be molded of a single type of plastic, it is desirable, for ease of tearing the weakened region, that the plastic be relatively brittle and not soft and tough. However, relatively brittle thermoplastic material, even when it still retains some of its elastomeric qualities, cannot easily be made deformable enough to act as its own gasket against the end of the neck of the container, and using a separate liner with such a closure makes it easier to achieve satisfactory sealing characteristics and, simultaneously, to achieve satisfactory tamper-evident operation.
Since it is important for closures to be usable with existing bottles, the configuration of the neck portion in common use must be taken into account in arranging the configuration of the pilfer-proof. A typical bottle has a neck with an external thread that extends from a point near the end a short distance toward the main body of the container and terminates at or near a bead that extends radically outwardly about as far as, or usually a little farther than, the outermost part of the thread. The bead has a shoulder that extends inwardly toward the axis of the neck and defines the end of the bead facing the main body of the container. Most, if not all, pilfer-proofs are expected to hook or contract under the shoulder.
On many bottles, there is a flange that extends outwardly from the neck at about the location where the neck begins to merge into the main body of the container. The axial distance between the shoulder of the bead and the flange limits the maximum axial length of the ring of the pilfer-proof and even though some containers do not have such a flange, it is commercially desirable to arrange closures to be as similar as possible and thus to accommodate a flange even when no flange is present. Thus, it is important to form the hooks on the pilfer-proof so that they are attached to the edge of the ring remote from the main part of the closure and so that they extend both upwardly toward the top of the closure and inwardly toward the axis to a region such that, when the closure is screwed onto a container, the tip of each hook will be at or very slightly below the shoulder of the bead so that the closure cannot be appreciably unscrewed without producing a large axial force due to pressure of the hooks on the shoulder.